A vacuum hopper loader, such as the proportioning hopper described and claimed in Evans et al U.S. Pat. No. 3,780,991, affords an efficient and economical apparatus for loading processing machines with granulated plastic and other similar free flowing materials. Commercial proportioning hopper loaders of this kind can readily provide for loading at high rates (e.g., 1,600 lbs/hour over short distances). Two or more materials can be effectively mixed together, by the hopper loader, in the course of the loading process.
Control of a hopper loader of this kind, however, often presents difficult problems. With previously known controls, a change of the desired proportions for the materials being mixed is often time-consuming, leading to undesirable down time for the hopper loader and the process equipment, such as a plastic molding press, fed by the hopper loader. Precise proportioning of the inlet materials and effective mixing present substantial problems, if conventional mechanical, electromechanical, or even electronic controls are utilized, often producing undesirable effects in the composition of the process apparatus output.
Furthermore, the previously known controls for hopper loaders of this type have not been readily adaptable to the requirements of auger grinders and other material sources that may require removal of a particular material, by the hopper loader, at a rate sufficient to avoid backup or other malfunction at the material source. Another problem with hopper loader controls results from variations in the hopper loader itself; construction of a control that is equally adaptable to a hopper loader using a local blower as a vacuum source and to one using a central blower for both vacuum and filter blowback presents substantial technical problems.